Rising fuel prices and global climate change concerns have revived the interest in renewable sources of energy. Using solar energy to grow photosynthetic microorganisms is one of the most attractive ways to produce transportation fuels. Successful implementation of biodiesel via seed crops is one example of employing plant-based photosynthesis for fuel production. However, recent assessments of crop-based fuel economy showed that it can lead to food stock deficiency and drive lifecycle emissions of greenhouse gases up through increased land usage. Utilization of photosynthetic microorganisms for primary biomass production has many advantages over growing crops. In particular, arid regions of the western U.S., for example, could be used for large-scale production excluding the competition with food-producing agriculture.
Cultivation of photoautotrophic microorganisms for metabolite and/or biomass production can be accomplished in various types of cultivation systems including open ponds and enclosed bioreactors. Each system has various advantages and limitations. Open ponds, for example, are designed to utilize natural sunlight while most of the enclosed bioreactor systems do require artificial illumination which results in additional energy expenditures. Open pond systems, however, are more prone to fouling by external contamination and are not suited to grow genetically modified organisms. In contrast, enclosed bioreactors provide highly controlled conditions, protection against external contamination, and higher growth rates and biomass/products yields while allowing use of genetically modified strains. Cultivation of photosynthetic organisms is also associated with several general problems which arise from the necessity to deliver CO2 into liquid medium and remove excess O2 produced as a result of photosynthesis in order to maintain desired growth conditions. The current practice is to continuously or periodically purge the system which adds significantly to the operating costs and results in frequent changes of cultivation conditions and reduction in efficiency. Removal of O2 by most other known methods such as by chemical catalysis is typically prohibitively costly. What is needed therefore is a solution that enables continuous operation under controlled conditions such as within an enclosed bioreactor without the need for venting as is required by the prior art. The present invention meets this need.